In recent years, a so-called monolithic circuit technology is becoming widespread in display devices of the active matrix type. The monolithic circuit technology is a technology in which pixel TFTs (Thin Film Transistors) for injecting charges to pixels and peripheral circuit TFTs constituting a peripheral circuit such as a driver circuit for driving scanning lines or signal lines connected to the pixel TFTs, are formed on the same glass substrate.
Relatively high voltage stress is applied to a gate electrode of a TFT used as an output transistor of a driver circuit included in this type of a display device. Due to the voltage stress, a gate threshold voltage (hereinafter referred to as a “threshold voltage”), which is one of electric characteristics of the TFT, shifts. The shift in threshold voltage of the TFT due to the voltage stress is caused by electrons being trapped in a gate insulating film of the TFT. When the threshold voltage of the TFT constituting the driver circuit remarkably shifts, it becomes impossible to sufficiently inject charge into pixels, thereby making it likely to deteriorate the display performance of the display device.
As a prior art directed to addressing this problem, Japanese Patent Application Laid-Open Publication No. 2006-174294 (Patent Document 1) describes a TFT having a double gate structure provided with a back gate to adjust a shift (fluctuation, variation) in threshold voltage of the TFT. According to this prior art, based on a relationship between a control voltage and a threshold voltage, which is previously obtained by experiments, a control voltage with the same polarity as that of a variation in threshold voltage is applied to the back gate, thereby adjusting the variation in threshold voltage of the TFT.